1. In the Side Menu, go to the Emails --> Add Email.
(Note we aren't going to send an email)
2. Scroll Down and click Add Page Content.
3. At the top of the page click on Templates.
4. Find a template you would like and click Set as Default Template.
5. Check the boxes of the email type you would like to use this template as.
For example you can check the box for "Add Email" and then going forward anytime you create an email from the Add Email page this template will be used.
6. Click the Save button.
Note that landing and content pages can be updated the same way.
To update the content / landing pages instead of clicking Add Email in step one click on Landing Pages --> View Landing Page instead.